Purification of titanium tetrachloride



July 5, 1955 B. ALPERT ET AL 2,712,523

PURIFICATION OF TITANIUM TETRACHLORIDE Filed June 12, 1951 IN VENTOR S MARSHALL B. ALBERT &WILLIAM F, SULLIVAN w w/ JL ATlvRNEY United States Patent C) PURIFICATION OF TITANIUM TETRACHLORIDE Marshall B. Alpert, Tompkinsville, N. Y., and William F. Sullivan, Roseland, N. J., assignors to National Lead Company, New York, N. Y., a corporation of New Jersey Application June 12, 1951, Serial No. 231,128

8 Claims. (Cl. 204-61) This invention relates to a method and apparatus for the purification of titanium tetrachloride.

It is very desirable to reduce the amount of impurities contained in commercial titanium tetrachloride, particularly to reduce the content of other metals. It has now been found that a very substantial reduction of such impurities can effectively be attained by the electrolytic reduction of titanium tetrachloride to lower chlorides, preferably mixtures of the trichloride and the dichloride, followed by recombination of the lower chlorides with chlorine, preferably from the reduction step.

The electrolytic reduction and recombination may advantageously be carried out in a bath of fused chlorides of the alkali and alkaline earth metals, including magnesium, in an electrolytic cell containing an anode and a cathode and a conduit for introducing titanium tetrachloride into the bath adjacent to the cathode. A hollow tubular cathode may advantageously be used for this purpose.

In carrying out the process of the invention the fused salts are admixed with lower titanium chlorides which can be prepared in any manner. The concentration of lower titanium chlorides in the fused salt bath is not critical but it is preferred that it be approximately one molal in the bath. The electrolyte preferably comprises a molten halide of at least one alkali-forming metal, that is, of the alkali and alkaline earth metals, including magnesium. Mixtures which form low melting point eutectics are most convenient.

It is also desirable although not essential to provide a gas barrier near the top portion of the anode to selectively collect the purified titanium tetrachloride which is released from the anode. The cathode may consist of a nickel or tantalum rod, or it may be a hollow tube through which the tetrachloride is introduced. The anode may consist of any suitable material, particularly carbon. An atmosphere of inert gas such as argon is preferably maintained above the fused salt bath in order to prevent contamination of the titanium tetrachloride by the outside atmosphere.

The titanium tetrachloride is introduced adjacent to the bottom of the cathode or through a hollow cathode and simultaneously a current of somewhat less than 2 faradays per mol of titanium tetrachloride is passed through the cell. The titanium tetrachloride is solubilized and reduced to titanium trichloride and titanium dichloride. A current of substantially more than two faradays per mol will produce a small amount of titanium metal which is undesirable. A current of one faraday per mol of titanium tetrachloride introduced will eventually produce a high concentration of titanium trichloride which is undesirable and tends to decrease the solubilization of the tetrachloride. Therefore currents substantially less than 2 are desirable and from about 1.1 to 1.6 are preferred.

The tetrachloride solubilizcd and reduced near the cathode is transferred throughout the solution by difiusion and convection and tetrachloride is reformed at the anode where it is released, condensed and collected as a purified product. Any current density up to 5.0 amperes per square centimeter may be utilized at the anode and the current density at the cathode should fall between 0.1 and 0.5. Particularly good results have been obtained using 0.2 amperes per square centimeter current density at both the anode and cathode.

The accompanying drawing is a sectional elevation'of an electrolytic cell embodying the principles of the invention.

In the drawing 10 is a suitable furnace provided with heating means such as gas burners 11. Disposed within the heated zone is cell container 12 which is made from a corrosion resistant material such as fused impervious silica and preferably protected externally by a metallic sheath 13 for instance of stainless steel or other metal resistant to the heat efiects involved. The cell container 12 is filled or partially filled with electrolyte 14 in which is suspended cathode 15 which preferably comprises a tube of a resistant metal, such as nickel or tantalum, lined for protection with silica liner tube 22. The anodes 16 may be formed of graphite rods and are submerged in the electrolyte. The upper portion of the cathode is surrounded by a barrier ring 17, for example of fused silica, dipping slightly below the surface of the bath 14. Cover 18 is provided to enclose the top of the cell and contains means 19 for admission of inert gas such as argon or helium to protect the surface of the molten electrolyte. The space above the electrode compartments is suitably enclosed as by cover 20 and provided with means as pipes 21 for carrying off the titanium tetrachloride reformed at the anode.

The following example is illustrative of the principles of the invention:

11,470 grams of a chloride salt electrolyte consisting of 7300 grams of strontium chloride, 2700 grams of sodium chloride, 240 grams of titanium dichloride and 1230 grams of titanium trichloride are placed in a cell of the type shown in the drawing and heated to 700 C. Vapors of titanium tetrachloride are added at the rate of 4.55 grams per minute through the hollow cathode into the fused salt bath. Simultaneously an electric current of 1.3 faradays per mol of titanium tetrachloride is supplied to the cell at 50 amperes with an impressed voltage of 1.5 volts. The titanium tetrachloride introduced at the cathode is solubilized and reduced to lower titanium chlorides. The lower chlorides move through the fused salt bath to the anode where titanium tetrachloride is again reformed and is released at the surface of the fused salt bath in the vicinity of the carbon anode where it is condensed and collected. An inert atmosphere of argon is maintained throughout the process above the fused bath in order to prevent contamination of the titanium values from the external atmosphere. Typically the cell operates at a cathode current density of about 0.2 amperes per square centimeter, an anode current density of about 1.0 ampere per square centimeter and a cell resistance of 0.03 ohms. Over a period of 6 hours, 1,620 grams of purified titanium tetrachloride are condensed and collected.

Typical spectrographic analysis of the original and the purified material are given in the table:

TABLE Speczrographic analysis (The designation as oxides or as metals is not intended to indlitaate the form in which the impurities exist in the materia 3 We claim: y 1. A method of purifying titanium tetrachloride which comprises passing into an electrolytic bath consisting of a a chloride of at least one alkali forming metal maintained in molten condition between aninsoluble anode and a cathode, impure titanium tetrachloride adjacent the cathode, passing an electric current of from about 1.1 to 1.6 faradays per'mole of titanium tetrachloride between the electrodes at an anode current density not exceeding about 5.0 amperes per square centimeter and a cathode current density between about 0.1 and 0.5 amperes per square centimeter, and withdrawing purified titanium tetrachloride from above the anode.

,2. A method as defined in claim 1 wherein the bath consists of a chloride of at least one alkali forming metal and at least one of the lower chlorides of titanium.

consists of a substantially eutectic mixture of chlorides of the alkali'forming metals and at least one of the lower chlorides of titanium.

V .5. A method as defined in claim 1 wherein the bath consists of a substantially eutectic mixture of chlorides of sodium and strontium.

6. A method as defined in claim 1 wherein the bath consists of a substantially eutectic mixture of chlorides of sodium and strontium and at least one of the lower chlorides of titanium. V

7. A method as defined in claim 1 wherein the anode and. cathode current densities are about 0.2 amperes per square centimeter.

8. A method as defined in claim 6 wherein the anode current density is about 1.0 ampere per square centimeter and the cathode current density is about 0.2 ampere's per square centimeter.

References Cited in the file of this patent UNITED STATES PATENTS v Spence Mar. 17, 1903 723,217 1,018,802 Acker Feb. 27, 1912 FOREIGN PATENTS Germany July 16, 1935 OTHER REFERENCES Electrolytic Oxidation and Reduction, by Glasstone I ct al.,' published in 1936, pages 46 and 106-108.

Chemical Abstracts, vol. 34 (1940), page 7756, ab-- 25 stract of article by Sklyarenko et al. 

1. A METHOD OF PURIFYING TITANIUM TETRACHLORIDE WHICH COMPRISES PASSING INTO AN ELECTROLYTIC BATH CONSISTING OF A CHLORIDE OF AT LEAST ONE ALKALI FORMING METAL MAINTAINED IN MOLTEN CONDITION BETWEEN AN INSOLUBLE ANODE AND A CATHODE, IMPURE TITANIUM TETRACHLORIDE ADJACENT THE CATHODE, PASSING AN ELECTRIC CURRENT OF FROM ABOUT 1.1 TO 1.6 FARADAYS PER MOLE OF TITANIUM TETRACHLORIDE BETWEEN THE ELECTRODES AT AN ANODE CURRENT DENSITY NOT EXCEEDING ABOUT 5.0 AMPERES PER SQUARE CENTIMETER AND A CATHODE CURRENT DENSITY BETWEEN ABOUT 0.1 AND 0.5 AMPERES PER SQUARE CENTIMETER, AND WITHDRAWING PURIFIED TITANIUM TETRACHLORIDE FROM ABOVE THE ANODE. 